This invention relates to a method and apparatus for the electrochemical determination of the oxygen concentration, particularly in biological matter, by means of an oxygen sensor with a measuring electrode and a counterelectrode.
In modern medicine, implantable heart pacemakers are used increasingly for the therapy of heart arrythmia. The heart muscle is stimulated by short current pulses which are transmitted by means of a stimulating electrode. The implantable heart pacemakers generally operate with a constant frequency and thus only imperfectly simulate the control of the healthy heart which changes its beat as required. Therefore, physiological control of the rate or frequency of heart pacemakers is desired, for which purpose, for instance, the oxygen concentration in the blood or tissue can be used as a control parameter. The frequency of the stimulating pulses is then adapted to the oxygen concentration or the partial oxygen pressure in order to make the heart beat faster if the oxygen supply is insufficient. For this purpose, the oxygen concentration must be measured continuously, i.e., an implantable oxygen sensor is required. Besides their use in heart pacemakers, oxygen sensors are also suitable for determining the oxygen content in respiration and can therefore be used in anesthetics and intensive patient monitoring.
Heretofore the oxygen concentration in the blood or tissue has been measured generally extracorporally but only to a very limited extent and only for a short time in vivo. As the basis for the measurement, the electrochemical principle, for which purpose a so-called Clark cell is employed has essentially been used heretofore (see U.S. Pat. Nos. 2,913,386; 3,260,656 and 4,076,496). In such an oxygen sensor, a diaphragm is arranged in front of the measuring electrode, whereby a diffusion limiting current adjusts itself at the electrode, i.e., a measuring signal proportional to the concentration is obtained.
The above-mentioned measuring methodology is not suited for long-term implantation since the limiting current depends heavily on the diffusion layer. Thus, the connective tissue layer, which is of necessity formed in the body after the implantation, can therefore falsify the measuring signal. In principle, the solution of this problem consists of using diaphrams which can be implanted over the long term. Diaphragms suited for this purpose, however, are not yet known to date.
It is an object of the invention to describe a method and apparatus which allows the determination of the oxygen concentration over long periods of time even if the sensor is implanted, using an oxygen sensor with a measuring and counterelectrode.